Chiral selectors classification

Different classifications for the chiral CSPs have been described. They are based on the chemical structure of the chiral selectors and on the chiral recognition mechanism involved. In this chapter we will use a classification based mainly on the chemical structure of the selectors. The selectors are classified in three groups (i) CSPs with low-molecular-weight selectors, such as Pirkle type CSPs, ionic and ligand exchange CSPs, (ii) CSPs with macrocyclic selectors, such as CDs, crown-ethers and macrocyclic antibiotics, and (iii) CSPs with macromolecular selectors, such as polysaccharides, synthetic polymers, molecular imprinted polymers and proteins. These different types of CSPs, frequently used for the analysis of chiral pharmaceuticals, are discussed in more detail later. 

In addition to the classification of liquid chromatographic enantioseparation methods by technical description, these methods could further be classified according to the chemical structure of the diverse CSPs. The chiral selector moiety varies from large molecules, based on natural or synthetic polymers in which the chirality may be based on chiral subunits (monomers) or intrinsically on the total structure (e.g., helicity or chiral cavity), to low molecular weight molecules which are irreversibly and/or covalently bound to a rigid hard matrix, most often silica gel. 

Over 100 stationary phases with immobilized chiral selectors are commercially available with further stationary phases described in the literature. Any attempt at a general classification into a reasonable number of groups with similar properties is not easy. Probably the most widely adopted classification scheme was proposed by Wainer, and is loosely based on the chiral recognition mechanism. Table 10.3 [61]. In the following sections, a different approach has been adopted based on the type of chiral selector. Coverage is incomplete, but includes all of the widely used stationary phases. 

Classification of immobilized chiral selectors (chiral stationary phases) by enantioselectivity mechanism TVpe Description... 

Today, a more pertinent classification should be proposed in accordance with the great number of chiral selectors reported in LC (about 1500 CSPs have been collected in ChirBase database) and our new knowledge of chiral separation mechanisms. For instance, chromatographic results are clearly suggesting that bimodal and supramolecular systems should be gathered quite separately. 

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